1. Field of the Invention
The present invention relates to a magnetic sensor, a method for manufacturing a magnetic sensor, and a current sensor including a magnetic sensor.
2. Description of the Related Art
In fields relating to, for example, motor driving techniques of electric cars and hybrid cars, since a relatively large current is used, a current sensor capable of measuring a large current in a non-contact manner has been required. As the current sensor described above, a sensor which uses a magnetic sensor detecting an induction magnetic field from a current to be measured has been known. As a magnetic detection element for a magnetic sensor, for example, a magnetoresistive effect element, such as a Giant magnetoresistance (GMR) element, may be mentioned.
A GMR element has as a basic structure, a laminate structure in which a fixed magnetic layer and a free magnetic layer are laminated to each other with a nonmagnetic material layer provided therebetween. The magnetization direction of the fixed magnetic layer is fixed in one direction by an exchange coupling bias by a laminate structure of an antiferromagnetic layer and a ferromagnetic layer or by the RKKY interaction (indirect exchange interaction) by a self-pinning structure in which two ferromagnetic layers are laminated to each other with a nonmagnetic interlayer provided therebetween. The magnetization direction of the free magnetic layer is configured to be changeable in accordance with an external magnetic field.
In a current sensor formed by using a magnetic sensor including a GMR element, since the induction magnetic field from a current to be measured is applied to the GMR element, the magnetization direction of the free magnetic layer is changed. Since the electric resistance of the GMR element is changed by the relationship between the magnetization direction of this free magnetic layer and the magnetization direction of the fixed magnetic layer, by the measurement of this electric resistance, the magnetization direction of the free magnetic layer can be detected. In addition, based on the magnetization direction detected by the magnetic sensor, the magnitude and the direction of the current to be measured which generates the induction magnetic field can be obtained.
Incidentally, in an electric car or a hybrid car, the drive of a motor is controlled based on a current value in some cases, or a method to control a battery is adjusted in accordance with a current value flowing thereinto in some cases. Hence, in order to more accurately detect a current value, a current sensor formed by using a magnetic sensor is required to improve the measurement accuracy of the magnetic sensor.
In order to improve the measurement accuracy of the magnetic sensor, for example, reduction in offset, reduction in variation of an output signal, and improvement in linearity (output linearity) are required to be realized. As one preferable method to respond to those requirements, reduction in hysteresis of the GMR element of the magnetic sensor may be mentioned. As a particular example of the method to reduce the hysteresis of the GMR element, there may be mentioned a method in which by application of a bias magnetic field to a free magnetic layer, the magnetization direction of the free magnetic layer is aligned even in the state in which the induction magnetic field from the current to be measured is not applied.
As a method to apply a bias magnetic field to a free magnetic layer, in International Publication No. 2012/081377, a method in which a hard bias layer formed of a permanent magnet is provided has been disclosed. In addition, in Japanese Unexamined Patent Application Publication No. 2012-185044, a method has been disclosed in which an antiferromagnetic layer capable of generating an exchange coupling bias with a free magnetic layer and of aligning the magnetization direction of the free magnetic layer in a predetermined direction in a magnetization changeable state is laminated on the free magnetic layer.